In grammar, a lexical category (also word class, lexical class, or in traditional grammar part of speech) is a linguistic category of words (or more precisely lexical items), which is generally defined by the syntactic or morphological behaviour of the lexical item in question. Common linguistic categories include noun and verb, among others. There are open word classes, which constantly acquire new members, and closed word classes, which acquire new members infrequently if at all.

Different languages may have different lexical categories, or they might associate different properties to the same one. For example, Japanese has as many as three classes of adjectives where English has one; Chinese, Korean and Japanese have classifiers while European languages do not grammaticalize these units of measurement (a pair of pants, a grain of rice); many languages don't have a distinction between adjectives and adverbs, adjectives and verbs (see stative verbs) or adjectives and nouns[citation needed], etc. Some argue that the formal distinctions between parts of speech must be made within the framework of a specific language or language family, and should not be carried over to other languages or language families.

Since the Greek grammarians of 2nd century BCE, parts of speech have been defined by morphological, syntactic and semantic criteria. However, there is currently no generally agreed-upon classification scheme that can apply to all languages, or even a set of criteria upon which such a scheme should be based.

Linguists recognize that the above list of eight word classes is drastically simplified and artificial.[5] For example, "adverb" is to some extent a catch-all class that includes words with many different functions. Some have even argued that the most basic of category distinctions, that of nouns and verbs, is unfounded,[6] or not applicable to certain languages.[7]

English frequently does not mark words as belonging to one part of speech or another. Words like neigh, break, outlaw, laser, microwave and telephone might all be either verb forms or nouns. Although -ly is an adverb marker, not all adverbs end in -ly and not all words ending in -ly are adverbs. For instance, tomorrow, fast, crosswise can all be adverbs, while early, friendly, ugly are all adjectives (though early can also function as an adverb).

In certain circumstances, even words with primarily grammatical functions can be used as verbs or nouns, as in "We must look to the hows and not just the whys" or "Miranda was to-ing and fro-ing and not paying attention".